I'm not sure what you men by "slightly opaque subdivisions",
I think he means the roads. They have no IG effect. Just map graphics. You'll see some around the northern lake, too.
but as egamma says, at the end of the Last Invasion, every region that was still blighted was scooped from the face of the earth, save for the cities, which were cast deep into the seas.
Tom left the cities on the map, just faded out, as if they are being viewed through 100+ feet of water. Sunken cities leaves the mystery of the lost and unknown, and is a nod to the history of the island.
